1. Field of the Invention
The present general inventive concept relates to a length reference bar system and method to test measuring equipment, and in particular, to a length reference bar system and method to test, calibrate, check calibration, and/or check stability during use of high precision instruments such as laser trackers, scanners and the like.
2. Description of the Related Art
A large number of industries require precise and accurate measuring for a number of applications such as production, manufacturing, and process control. In many such applications, measurement errors on the order of even one ten-thousandth of an inch can be critical. Instruments such as laser trackers, scanners and the like are particularly well suited for such applications because they provide extreme precision and accuracy. Nevertheless, as with any measuring device, laser trackers, scanners and the like must be calibrated and used properly to benefit from the high precision and accuracy capabilities of those instruments.
As is the case with calibration (or verifying proper calibration) of virtually any measuring instrument, checking calibration of a laser tracker or of a scanner is typically accomplished by measuring an object (such as a length reference bar) of a standard, known length and confirming that the instrument measures the appropriate length. In particular, such bars are used to verify whether a laser tracker instrument is yielding trustworthy results (i.e. demonstrating that it is properly calibrated) or used during a measurement job to establish whether anything has gone wrong during the course of the job. For example, a user of the instrument will measure the length reference bar at the beginning, middle, and end of a job. If the user obtains the same length measurement each time, the user will have a degree of comfort that the tracker has maintained a proper adjustment and/or position during the measurement.
Length bars are built and calibrated according to various specification standards such as governmental standards or engineering society standards. Unfortunately, most length reference bars are designed and calibrated for use only within a specified temperature range. This is due to the natural phenomenon of thermal expansion, which causes all materials to expand and contract when the materials are subjected to increases and decreases in temperature. Thus, laser tracker or scanner users must often check calibration of their instruments at temperatures substantially different than the environment in which they will be working, or must otherwise account for temperature variations when checking calibration. All alternatives are often impractical if not impossible, and furthermore tend to result in at least a slight degree of inaccuracy and/or imprecision.
Several length reference systems have been developed that include structural arrangements that are designed to counteract length increases caused by thermal expansion. For example, U.S. Pat. No. 6,836,323, the entire disclosure of which is incorporated herein by reference, discloses a length reference system which includes a compensating member that connects the wall of a bar to the wall of a probe attachment that slides within the bar. The compensating member is designed to increase in length by an amount equal to the combined length increase of the bar and the probe attachment. As the compensating member increases in length, it causes the probe attachment to slide inward in the hollow bar compensating for the length increase of the bar and the probe attachment. While this arrangement does cancel out the thermal expansion of the bar, it requires that the probe, or target which marks the point of reference for location by a laser tracker or scanner, be located along the axis of the length reference bar. As a result it is not possible to align a laser tracker's beam with the bar to use the inferometer alone to check calibration. Not doing so may lead to Az or El encoder contributions to error. Furthermore, the aligned-probe arrangement requires that any thermal expansion of the probe, as well as any members connecting the probe to the probe attachment, also be canceled out by the expansion of the compensating member.
Other length reference systems that have been designed to counteract length increases caused by thermal expansion are disclosed in U.S. Pat. Nos. 6,505,495 and 7,188,428, the entire disclosures of which are incorporated herein by reference. Like the system disclosed in U.S. Pat. No. 6,836,323, these systems include probes that are located axially with the length reference bar, preventing the ability to check calibration using the inferometer of a laser tracker alone and adding to the complexity of design and manufacturing of the reference bar.
Therefore, it is desirable to provide a length reference bar system and method that compensates for thermal expansion, tests whether an instrument is working properly using an inferometer alone, is easy to manufacture, and is simple to use.